The laser-active material described in U.S. Pat. No. 4,519,082 to Irwin Schneider is a lithium-doped potassium iodide (KI:Li) crystal containing point defects consisting of F.sub.2.sup.+ and (F.sub.2.sup.+).sub.A color centers. These color centers are produced in a crystal that is colored by ionizing radiation, annealed for several minutes near 250 degrees Kelvin (K) to form F.sub.2, (F.sub.2).sub.A, F.sub.2.sup.+, and (F.sub.2.sup.+).sub.A centers, and then irradiated near 148K with green light to convert F.sub.2 to (F.sub.2).sub.A centers and F.sub.2.sup.+ to (F.sub.2.sup.+).sub.A centers. Color centers are known in the applicable technology and are described in the technical literature. This material, however, is of limited use because its application involves a complex coloration process demanding an expensive particle accelerator to produce high-energy electrons and requires difficult handling procedures in that the material must be continuously stored at low temperatures to preserve the (F.sub.2.sup.+).sub.A centers, which would otherwise fade at high temperatures, such as room temperature (300K) The above limitations of the material have motivated the invention described herein below.
Additively-colored laser-active materials, such as sodium-doped potassium chloride (KCl:Na), and lithium doped potassium chloride (KC1:Li), are easily fabricated using a known enclosed-bomb technique; and laser-active (F.sub.2.sup.+).sub.A centers are created by exposing the crystalline materials to near-uv or visible light. The term "additive coloration" as used herein refers to this advantageous technique that is used in the practice of the invention. Color centers in additively-colored crystals are long lived at room temperature. One might expect that the technique used to prepare any of the above mentioned crystals could be successfully applied to the formation of (F.sub.2.sup.+).sub.A centers in additively-colored KI:Li (or RbI:Li) crystals, so that one would regard KI:Li (or RbI:Li) crystals containing (F.sub.2.sup.+).sub.A centers to be desirable for use as laser-active materials. Unfortunately, all previous attempts to form (F.sub.2.sup.+).sub.A centers in additively-colored KI:Li have failed for causes unrecognized until recently when we produced these laser-active centers for the first time in "additively-colored" KI:Li crystals.
Our discovery that nitrogen is a serious and previously unsuspected contaminant which diffuses into the KI or RbI lattice at elevated temperatures, especially during the additive coloration process, and chemically combines with isolated lithium in the lattice inhibiting the formation of (F.sub.2.sup.+).sub.a centers, has led us to devise a process for additively coloring KI:Li and RbI:Li crystals.
Lithium-(F.sub.2.sup.+).sub.A centers in potassium iodide are laser-active having a tuning range from 2.4 to 4.0 microns. The tuning range for these centers in rubidium iodide is predicted to extend from 2.6 to 4.2 microns. This wavelength range in the infrared spectral region is important in molecular photochemistry and spectroscopy since the range covers several fundamental stretch frequencies including the carbon-hydrogen (CH) and oxygen-hydrogen (OH) modes of organic and other materials. The lithium-doped potassium iodide laser-active material reported elsewhere by Irwin Schneider and Charles Marquardt in the technical literature is of limited use because the material must be "colored" at low temperatures with high-energy electrons to create various kinds of color centers subsequent to optical processing with visible electromagnetic energy, and requires continuous storage at low temperatures to preserve the laser-active (F.sub.2.sup.+).sub.A centers as well as all other color centers. In contrast, "additively-colored" laser materials, for example, sodium-doped potassium chloride (KCl:Na) and lithium-doped potassium chloride (KCl:Li) that are "colored" at high temperature in the presence of an alkali metal vapor, do not require a source of high-energy electrons for coloration, and are easily colored through a simple thermal process Also, once colored, these materials show an indefinitely-long shelf-life near room temperature. Hence, the production of additively-colored KI:Li and RbI:Li crystals has evident advantages that become attainable through practice of the invention.